Thin faces warning 3Dprint tools
There are two ways for you to handle this.

The first is through the length unit settings in the scene tab of the properties panel (bottom right in the image below). You can simply set the scene units to millimeters instead of meters (which is the default), and the 3D printing toolbox tests will update accordingly.

The other one is by changing the thickness threshold in the 3D printing toolbox thickness tests to whichever is the limit you're aiming for (which corresponds to your 3D printing tools and materials).